U.S. Pat. Nos. 3,814,091 and 3,901,230 disclose anesthesia rebreathing systems characterized by a geometry which preferentially vents expired alveolar gas, rich in carbon dioxide (CO.sub.2), while retaining fresh gas and initially expired dead space gas, rich in oxygen (O.sub.2), for rebreathing by the patient to thus minimize the need for CO.sub.2 absorption. More particularly, the systems comprise a patient circuit incorporating an overflow tube whose entrance is located very close to the patient. The overflow tube exits at a patient overflow (commonly referred to as "Pop-Off") valve which is located close to an anesthesia machine where it can be conveniently controlled by an attending anesthetist. By locating the overflow tube entrance close to the patient, it functions to preferentially vent alveolar gas through the patient overflow (i.e. Pop-Off) valve and save dead space and unbreathed gas within the tubing and reservoir of the patient circuit. The Pop-Off valve is operable in two different modes, i.e., (1) as a manually controlled variable orifice for spontaneous, manually assisted or controlled ventilation and (2) as an automatically Controlled Valve responding to a positive control pressure for manually assisted or controlled ventilation or mechanically controlled ventilation.
The enhanced system described in U.S. Pat. No. 3,901,230 can be viewed as functionally including (1) a patient circuit and (2) a ventilator/isolator (V/I) circuit for controlling gas volume and pressure in the patient circuit. The preferred embodiment can be viewed as structurally including (1) a single use portion and (2) a reusable portion. The embodiment is configured so that the single use portion forms most of the patient circuit with the reusable portion forming the V/I circuit and part of the patient circuit, e.g. the patient Pop-Off Valve. The V/I circuit includes a constant volume (e.g. rigid) container (forming part of the system's reusable portion ) within which a variable volume patient breathing reservoir (e.g. a flaccid bag) (forming part of the single use portion ) is accommodated. The pressure within the rigid container is controlled (1) during manually assisted or controlled ventilation, by an attending anesthetist squeezing an outside bag and (2) during mechanically controlled ventilation by a conventional MECHANICAL VENTILATOR. The pressure variations in the rigid container are applied to the patient circuit via the flexible walls of the patient bag. Crosscontamination is eliminated in the disclosed preferred embodiment because the patient expired gas cannot come into contact with the reusable portion components exposed to inspired gas.
U.S. Pat. No. 4,991,576 discloses an anesthesia rebreathing system which retains the advantages of the systems disclosed in U.S. Pat. Nos. 3,814,091 and 3,901,230 and which incorporates additional features to enhance ease of use and safe operation.
In accordance with U.S. Pat. No. 4,991,576, instead of using multiple independently operable user controls, an integrated control knob is provided whose position determines the system operating mode. Thus, in a first disclosed embodiment, the user control knob can be rotated to any one of the following mutually exclusive positions:
1. AUXILIARY OUTLET PA1 2. OFF PA1 3. MECHANICAL VENTILATOR MODE PA1 4. MANUAL BAG MODE
Position 3 is used to ventilate the patient with a MECHANICAL VENTILATOR. Position 4 is used to allow the patient to breath spontaneously or to manually control or assist ventilation. In a second disclosed embodiment, the four aforementioned positions are supplemented by an automatic bag mode position (i.e. position 5) at which the system self adjusts without requiring Manual Valve adjustment of a V/I Circuit Overflow Valve during spontaneous and manually assisted or controlled ventilation.
The first and second embodiments of U.S. Pat. No. 4,991,576 additionally differ from one another in that the first embodiment uses overflow gas from the patient circuit as working gas for the V/I circuit. In the second embodiment, V/I working gas is derived from a high pressure gas source (preferably dry medical grade oxygen).
The preferred single use portion of U.S. Pat. No. 4,991,576 includes a connector body configured to be detachably seated on the reusable portion for operative mating to the V/I circuit. A sensor means is coupled to the aforementioned control knob for preventing it from being moved to any of the aforementioned ventilating mode positions unless the connector body is properly seated on the reusable portion. Latching means configured to allow the connector body to be readily manually seated prevent it from being inadvertently unseated while the system is in a ventilating mode. In order to unseat (i.e., detach) the latched connector body, an unlatching means, operably coupled to the control means and actuatable only when the control knob is not in a ventilating mode position, e.g. the off position, functions to both unlatch and eject the connector body.